Introduction
The Boeing X-37 (also known as the X-37 Orbital Test Vehicle) is an American reusable unmanned spacecraft. It is boosted into space by a rocket, then re-enters Earth’s atmosphere and lands as a spaceplane. The X-37 is operated by the United States Air Force for orbital spaceflight missions intended to demonstrate reusable space technologies. It is a 120%-scaled derivative of the earlier Boeing X-40.

The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. It conducted its first flight as a drop test on 7 April 2006, at Edwards Air Force Base, California. The spaceplane’s first orbital mission, USA-212, was launched on 22 April 2010 using an Atlas V rocket. Its successful return to Earth on 3 December 2010 was the first test of the vehicle’s heat shield and hypersonic aerodynamic handling. A second X-37 was launched on 5 March 2011, with the mission designation USA-226; it returned to Earth on 16 June 2012. A third X-37 mission, USA-240, launched successfully on 11 December 2012.

Development

Origins
In 1999, NASA selected Boeing Integrated Defense Systems to design and develop an orbital vehicle, built by the California branch of Boeing’s Phantom Works. Over a four-year period, a total of $192 million was contributed to the project, with NASA contributing $109 million, the U.S. Air Force $16 million, and Boeing $67 million. In late 2002, a new $301-million contract was awarded to Boeing as part of NASA’s Space Launch Initiative framework.

The X-37 was transferred from NASA to the Defense Advanced Research Projects Agency (DARPA) on 13 September 2004. Thereafter, the program became a classified project, although it is not known whether DARPA will maintain this status indefinitely. DARPA promoted the X-37 as part of the independent space policy that the United States Department of Defense has pursued since the 1986 Challenger disaster.

The X-37 was originally designed to be carried into orbit in the Space Shuttle’s cargo bay, but underwent redesign for launch on a Delta IV or comparable rocket after it was determined that a shuttle flight would be uneconomical. The X-37’s aerodynamic design was derived from the Space Shuttle, hence the X-37 has a similar lift-to-drag ratio, and a lower cross range at higher altitudes and Mach numbers compared to DARPA’s Hypersonic Technology Vehicle.

As part of its mission goals, the X-37 was designed to rendezvous with friendly satellites to refuel them, or to replace failed solar arrays using a robotic arm. Its payload could also support Space Control (Defensive Counter-Space, Offensive Counter-Space), Force Enhancement and Force Application systems. An early requirement for the spacecraft called for a delta-v of 7,000 mph (3.1 km/s) to change its orbit.

Glide testing
The vehicle that was used as an atmospheric drop test glider had no propulsion system. Instead of an operational vehicle’s payload bay doors, it had an enclosed and reinforced upper fuselage structure to allow it to be mated with a mothership. In September 2004, DARPA announced that for its initial atmospheric drop tests the X-37 would be launched from the Scaled Composites White Knight, a high-altitude research aircraft.

On 21 June 2005, the X-37A completed a captive-carry flight underneath the White Knight from Mojave Spaceport in Mojave, California. Through the second half of 2005, the X-37A underwent structural upgrades, including the reinforcement of its nose wheel supports. Further captive-carry flight tests and the first drop test were initially expected to occur in mid-February 2006. The X-37’s public debut was scheduled for its first free flight on 10 March 2006, but was canceled due to an Arctic storm. The next flight attempt, on 15 March 2006, was canceled due to high winds.

On 24 March 2006, the X-37 flew again, but a datalink failure prevented a free flight, and the vehicle returned to the ground still attached to its White Knight carrier aircraft. On 7 April 2006, the X-37 made its first free glide flight. During landing, the vehicle overran the runway and sustained minor damage. Following the vehicle’s extended downtime for repairs, the program moved from Mojave to Air Force Plant 42 (KPMD) in Palmdale, California for the remainder of the flight test program. White Knight continued to be based at Mojave, and was ferried to Plant 42 when test flights were scheduled. Five additional flights were performed, two of which resulted in X-37 releases with successful landings. These two free flights occurred on 18 August 2006 and 26 September 2006.

On 17 November 2006, the U.S. Air Force announced that it would develop its own variant from NASA’s X-37A. The Air Force version was designated the X-37B Orbital Test Vehicle (OTV). The OTV program was built on earlier industry and government efforts by DARPA, NASA and the Air Force, and was led by the U.S. Air Force Rapid Capabilities Office, in partnership with NASA and the Air Force Research Laboratory. Boeing was the prime contractor for the OTV program. The X-37B was designed to remain in orbit for up to 270 days at a time. The Secretary of the Air Force stated that the OTV program would focus on “risk reduction, experimentation, and operational concept development for reusable space vehicle technologies, in support of long-term developmental space objectives.”

The X-37B was originally scheduled for launch in the payload bay of the Space Shuttle, but following the 2003 Columbia disaster, it was transferred to a Delta II 7920. The X-37B was subsequently transferred to a shrouded configuration on the Atlas V rocket, following concerns over the unshrouded spacecraft’s aerodynamic properties during launch. Following their missions, X-37B spacecraft land on a runway at Vandenberg Air Force Base, California, with Edwards Air Force Base as an alternate site. In 2010, manufacturing work began on the second X-37B, OTV-2, which was first launched in March 2011.

Design
The X-37 Orbital Test Vehicle is a reusable robotic spaceplane. It is a 120%-scale derivative of the Boeing X-40, measuring over 29 feet (8.8 m) in length, and features two angled tail fins. The X-37 launches atop an Atlas V version 501 rocket with a Centaur second stage. The X-37 is designed to operate in a speed range of up to Mach 25 on its reentry.

Among the technologies demonstrated in the X-37 include an improved thermal protection system, enhanced avionics, an autonomous guidance system and an advanced airframe. The spaceplane’s thermal protection system is built upon previous generations of atmospheric reentry spacecraft, incorporating silica ceramic tiles. The X-37’s avionics suite was used by Boeing to develop its CST-100 manned spacecraft. According to NASA, the development of the X-37 will “aid in the design and development of NASA’s Orbital Space Plane, designed to provide a crew rescue and crew transport capability to and from the International Space Station”.

The X-37 is independently powered by one Aerojet AR2-3 engine using storable propellants, providing thrust of 6,600 pounds-force (29.341 kN). The human-rated AR2-3 engine had been used on the dual-power NF-104A astronaut training vehicle, and was given a new flight certification for use on the X-37 with hydrogen peroxide/JP-8 propellants.

The X-37 lands automatically upon returning from orbit, and is the second reusable spacecraft to have such a capability, after the Soviet Buran shuttle. The X-37 is the smallest and lightest orbital spaceplane flown to date; it is approximately a quarter the size of the Space Shuttle orbiter.

Operational history

OTV-1
OTV-1, the first X-37B, launched on its first mission – USA-212 – on an Atlas V rocket from Cape Canaveral Air Force Station, Florida, on 22 April 2010 at 23:58 GMT. The spacecraft was placed into low Earth orbit for testing. While the U.S. Air Force revealed few orbital details of the mission, amateur astronomers claimed to have identified the spacecraft in orbit and shared their findings. A worldwide network of amateur astronomers reported that, on 22 May 2010, the spacecraft was in an inclination of 39.99 degrees, circling the Earth once every 90 minutes on an orbit 249 by 262 miles (401 by 422 km). OTV-1 reputedly passed over the same given spot on Earth every four days, and operated at an altitude of 255 miles (410 km), which is typical for military surveillance satellites. Such an orbit is also common among civilian LEO satellites, and the spaceplane’s altitude was the same as that of the ISS and most other manned spacecraft.

The U.S. Air Force announced on 30 November 2010 that OTV-1 would return for a landing during the 3–6 December timeframe. As scheduled, OTV-1 de-orbited, reentered Earth’s atmosphere, and landed successfully at Vandenberg AFB on 3 December 2010, at 1:16 PST (09:16 UTC), conducting America’s first autonomous orbital landing onto a runway; the first spacecraft to perform such a feat was the Soviet Buran shuttle in 1988. In all, OTV-1 spent 224 days in space. OTV-1 suffered a tire blowout during landing and sustained minor damage to its underside.

OTV-2
OTV-2, the second X-37B, launched on its inaugural mission, designated USA-226,aboard an Atlas V rocket from Cape Canaveral on 5 March 2011. The mission was classified and described by the U.S. military as an effort to test new space technologies. On 29 November 2011, the U.S. Air Force announced that it would extend the mission of USA-226 beyond the 270-day baseline design duration. In April 2012, General William L. Shelton of the Air Force Space Command declared the ongoing mission a “spectacular success”.

On 30 May 2012, the Air Force stated that OTV-2 would complete its mission and land at Vandenberg AFB in June 2012. The spacecraft landed autonomously on 16 June 2012, having spent 469 days in space.

OTV-3
OTV-3, the second mission for the first X-37B and the third X-37B mission overall, was originally scheduled to launch on 25 October 2012, but was postponed because of an engine issue with the Atlas V launch vehicle. The X-37B was successfully launched from Cape Canaveral on 11 December 2012. The classified mission is designated USA-240 and is expected to last several months.

Controversies
In 2010, Tom Burghardt wrote for Space Daily that the X-37B could be used as a spy satellite or to deliver weapons from space. The Pentagon has denied claims that the X-37B’s mission supports the development of space-based weapons. In January 2012, allegations were made that the X-37B was being used to spy on China’s Tiangong-1 space station module. Former U.S. Air Force orbital analyst Brian Weeden later refuted this claim, emphasizing that the different orbits of the two spacecraft precluded any practical surveillance fly-bys.

VariantsX-37A
The X-37A was the initial NASA version of the spacecraft; the X-37A Approach and Landing Test Vehicle (ALTV) was used in drop glide tests in 2005 and 2006.

X-37B
The X-37B is a modified version of the NASA X-37A, intended for the U.S. Air Force.[3] It conducted orbital test missions in 2010, 2011 and 2012.

X-37C
In 2011, Boeing announced plans for a scaled-up variant of the X-37B, referring to the spacecraft as the X-37C. The X-37C would be between 165% and 180% larger than the X-37B, allowing it to transport up to six astronauts inside a pressurized compartment housed in the cargo bay. Its proposed launch vehicle is the Atlas V Evolved Expendable Launch Vehicle.[64] The X-37C may compete with Boeing’s CST-100 commercial space capsule.[65]